New precursors for ceramic fibers that lead to pure silicon carbide, SiC, and do not thermally degrade, or do not need oxygen curing are of great commercial interest.
The most commonly used polycarbosilane precursor to silicon carbide ceramics is "Nicalon", manufactured by the Nippon Carbon Company. (S. Yajima, Am. Ceram. Soc. Bul., 62,893 (1983)). "Nicalon" is based on polydimethylsilane [Si(CH.sub.3).sub.2 ].sub.n, a polymer which has a silicon to carbon ratio (Si:C) of 1:2. In order to manufacture ceramic fibers from a ceramic precursor, it must retain its shape during firing. To accomplish this with "Nicalon", firing must be carried out in the presence of oxygen which stabilizes the "Nicalon" preceramic. This leads to an undesirably high level of oxygen in the final ceramic resulting in poor resistance to thermal degradation in use. As expected, an analysis of the ceramic fiber showed both a carbon-rich state and a high level of oxygen. The composition corresponded to: SiC+0.78C+0.22 SiO.sub.2. As a further disadvantage, "Nicalon" undergoes an undesirably large 30-40% weight loss during pyrolysis to the ceramic.
There are a variety of other polysilane and polycarbosilane polymers which yield non-stoichiometric SiC with significant weight loss.
European Patent Application No 139257, discloses the synthesis of a polycarbosilane that yields silicon carbide after pyrolysis at 1200.degree. C. with a weight loss of 42%.
Schilling et al., Am. Ceram. Soc. Bul., 62, 912 (1983), report the synthesis of a number of polycarbosilanes which produce silicon carbide with a significant weight loss after firing.
C. L. Czekaj et al., J. Am. Ceram. Soc., 73, 352 (1990), report the reaction of Cl.sub.2 CH.sub.3 SiCH.sub.2 Cl with magnesium followed by lithium aluminum hydride reduction to yield a polycarbosilane with a Si:C ratio of 1:2.
It is an object of the present invention to provide a polycarbosilane ceramic precursor that gives a minimum weight loss during firing. It is a further object of the present invention to provide a novel polycarbosilane that has a Si:C ratio of essentially 1:1. It is a further object of the present invention to provide a novel polycarbosilane ceramic precursor that is air and water stable. It is a further object of the present invention to provide a novel polycarbosilane with which there is no need to add oxygen to retain the shape of pulled fibers during firing.